A Digital Subscriber Line (xDSL) technology is a technology that provides broadband service by utilizing existing copper wire resource. The application of the xDSL technology can effectively protect the investment of the operators and provide the broadband service of high quality to the subscriber. Consequently, the xDSL technology has been briskly developed throughout the world since it is known. The data rate on the downlink from the Central Office (CO) to the Customer Premise Equipment (CPE) is much greater than that on the uplink from the CPE to the CO by utilizing the Asymmetric Digital Subscriber Line (ADSL) technology in the xDSL, which is suitable for the asymmetric feature of data traffic of residential broadband access service. Therefore, the ADSL technology is prevailing in residential broadband access application.
The frequency used by the ADSL is much higher than that of the conventional telephone signal. The frequency band of the ADSL is 25.875 kHz-1104 kHz. The frequency band of ADSL2+ is 25.875 kHz-2208 kHz. Because the ADSL and the Plain Old Telephone Service (POTS) occupy different frequency bands, they may transmit on a same telephone line simultaneously and may be separated by a simple splitter at the receiving and transmitting sides. A device for providing a centralized multiple ADSL services is called DSL Access Multiplexer (DSLAM).
The coverage of common ADSL signals is about 5 km (18 kft). The link between the CO and the CPE can not be established because of the attenuation of the ADSL signals in the downlink when the loop goes beyond 5 km, thereby influencing the popularization of the ADSL. Consequently, the DSLAM is generally moved down to be closer to the subscribers, which may greatly increase the operational cost of the operators. In order to extend the coverage of the ADSL signals, two solutions are given in the prior art.
One solution is that the Reach Extended ADSL2 (READSL2), i.e., ADSL2 annex L, is proposed in ADSL2 for providing ADSL service on a long loop, and the coverage of the ADSL signals is extended up to 22 kft. However, READSL2 requires that equipment at both sides of the communication support ADSL2, millions of existing ADSL lines cannot be used for the long loop of the ADSL by adopting READSL2.
The other solution is that the coverage of the ADSL signals is extended by some subscribers based on proprietary protocols. For example, the frequency band of 25 kHz-138 kHz is used for the downlink transmission by adopting an echo cancellation (EC) method. The coverage of the ADSL signals is extended substantially owing to the less attenuation of the low-frequency signal. However, the proprietary protocol can't be interoperable with a common modem using Frequency-Division Multiplexing (FDM) mode. If EC method is applied to the environment with high service penetration rate, the achievable data rate or reach may be degraded dramatically due to the serious near end crosstalk.